Apparatus for growing of epitaxial layers

ABSTRACT

A holder for liquid epitaxial growth includes a vertically slotted wall and a plurality of stacked recessed receptacles closely fitted within the holder and adapted to support the substrates. Each support is slotted through the peripheral wall thereof whereby to communicate the recess with the vertical slot. The loaded holder is contained in a crucible and pressurized means charge the crucible with the growth material.

United States Patent [191 Springthorpe [451 May 7,1974

1 1 APPARATUS FOR GROWING OF EPITAXIAL LAYERS [75] Inventor: Anthony John Springthorpe, Richmond, Ontario, Canada [73] Assignee: Bell Canada-Northern Electric Research Limited, Ottawa, Ontario, Canada 22 Filed: Oct.24, 1972 21 Appl.No.: 299,906

Related US. Application Data [63] Continuation-impart of Ser. No. 128,284, March 26,

1971, abandoned.

[52] US. Cl. 118/429, 118/500 [51] Int. Cl. B05c 3/109 [58] Field of Search 148/171; 118/500, 429,

[56] References Cited UNITED STATES PATENTS 1,404,309 1/1922 McDonald 118/425 X Hanson 118/421 X 2,800,102 7/1957 Weiskopf et al 118/500 X 2,837,055 6/1958 Whitehead 118/500 3,168,100 2/1965 Rich 118/500 X 3,461,842 8/1969 Conrad et a1. 118/500 3,576,176 9/1969 Pickett 118/500 3,771,490 2/1972 Kinney et al 118/421 X Primary ExaminerMorris Kaplan Attorney, Agent, or Firm-Sidney T. Jelly [57] ABSTRACT A holder for liquid epitaxial growth includes a vertically slotted wall and a plurality of stacked recessed receptacles closely fitted within the holder and adapted to support the substrates. Each support is slotted through the peripheral wall thereof whereby to communicate the recess with the vertical slot. The loaded holder is contained in a crucible and pressurized means charge the crucible with the growth material.

3 Claims, 10 Drawing Figures PATENTED MY 7 I974 SHEET 2 1F 3 PATENTEDIAY 1 m4 SHEET '3 BF 3 Fig. IO

APPARATUS FOR GROWING OF EPITAXIAL LAYERS This application is a continuation-in-part application of US. application Ser. No. l28,284 filed Mar. 26, 1971, now abandoned and entitled Growing'of Epitaxial Layers on Substrates.

This invention relates to the growing of epitaxial layers on substrates by the immersion of substrate crystals in a saturated solution, the crystals in the form of slices or wafers.

Growth of epitaxial layers from a liquid phase, referred to as liquid phase epitaxy, is a technique which enables high quality single crystal layers of semiconducting and insulating material to be prepared. Such layers find application in active and passive solid state devices, depending upon the material used. Some examples of applications are Tunnel Diodes, Gunn microwave oscillators, Bubble Domain memories, and

light emitting diodes. Liquid phase epitaxy has been found to be a very satisfactory method of producing the active layers and light emitting junctions for light emitting diodes.

Various techniques exist for producing layers by liquid phase epitaxy. In one such technique, referred to as horizontal liquid phase epitaxy, a saturated solution of a compound in a suitable solvent is poured onto a single crystal substrate slice or wafer, followed by slow cooling of the whole system. The dissolved compound crystallizes on the immersed substrate as an epitaxial layer. By varying the volume of the melt and the rate of cooling different thicknesses of growth can be obtained, for example (Ll-I mils. If the doping of the melt differs from that of the substrate, p-n junction structure can be easily fabricated. Generally, the substrate is positioned at one end of a crucible, which is held horizontal in a furnace. A pool of molten solution is created at the other end of the crucible and at the appropriate temperature the crucible is tilted slightly causing the molten solution to flow over the substrate. Controlled slow cooling causes epitaxial growth on the substrate.

Another technique is one of whicha substrate crystal slice or wafer is attached to a vertical holder, the substrate held on one edge, and then immersed in a saturated solution. Growth is initiated by controlled slow cooling.

A further technique comprises floating a substrate crystal onto the surface of a saturated solution. Cooling again causes epitaxial growth.

Various disadvantages exist in the various known techniques. Only one substrate crystal can be processed at one time. In the horizontal techniques in which the crucible is tilted, this generally requires tilting of the entire furnace. The necessity to tilt is inconvenient. In the vertical technique, the substrate attached to a holder, convection currents in the molten solution can arise due to the holder and give rise to poor quality layers. Rigid support of the substrate is necessary and some of the substrate surface is obscured, reducing yield, and an expitaxial layer is formed on both surfaces of the substrate. The floating technique suffers from the disadvantages that solution convection can cause striations on the epitaxial surface. The holder used to lower the substrate onto the solution surface influences solution convection currents and causes areas of heavier growth; many solutions have thin oxide skins on the surface which will interfere with the wetting of the substrate crystal and inhibit eptiaxial growth; and if the solute crystals, which nucleate in the solution, are less dense than the solvent they will float and, coming in contact with the growing epitaxial layer, interfere with the growth.

The present invention provides an apparatus, for

" growing epitaxial layers, by which large numbers of substrates can be processed at one time; can provide conditions in which each substrate is in a convection free environment; enables clean solution to cover the substrates without wetting difficulties due to oxide layers; avoids interference by crystallites nucleated in the solution; provides some control over the epitaxial layer thickness; exposes all of one surface of the substrate to epitaxial growth; can be used for large area flat epitaxial growths; and the formation of abrupt p-n junctions. Further, by use of a particular method, with associated apparatus, the amount of solution required per slice can be reduced, providing a cost reduction.

According to one feature of the invention there is provided an apparatus for the growing of an epitaxial layer on one surface of each of a plurality of substrates which, in the broadest concept, comprises mounting a plurality of substrates in a holder in spaced parallel relationship; heating the holder and substrates to the saturated solution temperature; immersing holder and substrates in the solution; and slowly cooling the solution in contact with the substrates to cause epitaxial growth on the substrates surfaces. Immersion can be obtained by lowering the holder into the solution, or by causing the solution to flow into the chamber in which is positioned the holder.

In accordance with another feature of the invention there is provided apparatus for the growing of an epitaxial layer on one surface of each of a plurality of substrates comprising a holder having a plurality of supports each support adapted for the mounting of a substrate. In particular the supports are in the form of plates, and one of the surfaces of the plates may be recessed for the reception of the substrates.

The invention will be understood by the following description of certain processes, and various forms of apparatus, by way of example, in conjunction with the accompanying drawings, in which:

FIG. 1 is a vertical cross-section through a holder having several recessed support plates;

FIG. 2 is a plan view of a support with a square recess;

FIG. 3 is a plan view of a support with a circular recess;

FIG. 4 is a diagrammatic vertical cross-section through a furnace, with a solution container and substrate holder in position;

FIG. 5 is a cross-section through a further form of apparatus;

FIG. 6 is a cross-section of a holder for the apparatus of FIG. 5, on the line VI-VI of FIG. 7;

FIG. 7 is a plan view of the holder of FIG. 6;

FIG. 8 is a cross-section of a recessed support plate as used in the holder of FIGS. 6 and 7, on the line VIII- VIII of FIG. 9;

FIG. 9 is a plan view of the support plate of FIG. 8, and

FIG. 10 is a cross-section through an assembly of holder, support plates and substrates.

FIG. 1 illustrates a holder 20 having a plurality of support members or plates 21, each having a recess 22.

The support members 21 are attached to the spaced apart arms 23 forming the lower part of the holder 20. A substrate crystal 24 is placed in each recess 22.

. The recesses 22 have a shape convenient for the shape of substrate .crystal to be processed. Thus, as illustrated in FIG. 2, the recesses 22 can be rectangular for rectangular substrates, or as in FIG. 3, circular plate 21 has acircular recess 22' for circular substrates.

FIG. 4 illustrates diagrammatically a furnace 30 heated in the present example, by electric elements 31. Other forms of heating, for example RF heating, can be used. The furnace is closed at the lowest end by a lower plug 32. The crucible 33 rests on the lower plug 32 and contains a molten saturated solution 34. The top of the furnace is closed by a further plug 35 having a hole 36, the lower end 38 of the holder, having support member 39, being immersed in the solution 34. A controlled atmosphere is maintained in the furnace, as is usual, for' example by means of a gas inlet.

A typical process is as follows. A saturated solution of the material to be grown on the substrates is prepared, in the furnace 30; the furnace temperator stabilized at a temperature slightly above the nucleation temperature. Substrate crystals are positioned on the support members 39 of the holder 37 and the holder and support members with substrates heated to the temperature of the solution 34, and the holder is inserted in the furnace to immerse the substrates in'the solution. Controlled slow cooling of the furnace then follows, with epitaxial growth occurring on one surface of each of the substrates. After a predetermined time the holder is removed, and the substrates with their grown epitaxial layers removed from the support members. The substrates are then further processed in accordance with their intended use, in a known manner.

. GaP, the distance is equal to or less than mm.

The bottom of each support member acts to skim the surface of the solution of oxide layers, as it pours onto 3 the substrates, to avoid wetting. problems. Since the substrates are at the bottom of their growth cells no crystallites nucleated in the solution will interfere with the epitaxial growth. All of the crystal surface is in contact with the solution. By altering the distance between support members, to vary the amount of solution in contact-with the substrate, some control is obtained of the thickness of the epitaxial layer for a fixed rate of cooling. Large epitaxial layers can be grown and abrupt p-n junctions can be obtained.

' Large numbers of substrates can be processed at one time. For example a stack of support members separated by 2.5mm can be accomodated in a length less than 2.5 inches.

Alternative ways of controlling the thickness of the epitaxial layer can be used. For example, the holder can be withdrawn from the solution after cooling through a small temperature range. The substrates can be slightly inclined to the horizontal-typically up to 20to cause the solution to run off the substrates as the holder is removed. In another alternative, the

holder is dipped into the solution and withdrawn with a small amount of solution resting on each of the substrates. The holder would then be rapidly moved to a cooler part of the system giving rapid growth and yielding layers much thinner than those normally obtained.

When the required distance between support memhers-for thickness controlis less than which it is considered would give easily flowing access to the solutionfor example with rather viscous solutions, it is possible to mount the support members on the holder so that they can slide apart vertically. The holder is immersed in the solution with the support members widely spaced and then the support members are moved to the desired spacing.

FIGS. 5 to 10 illustrates apparatus for an alternative method for growing the epitaxial layers. In the arrangements so far described, the holder is inserted into the crucible, while the crucible contains the solution or melt. This requires that the clearances between holder and crucible be such that easy insertion and removal of the holder is possible. It will be appreciated that the holder is positioned in the crucible while the latter is in the furnace and this is a somewhat difficult procedure. The clearance between holder and crucible represents a dead volume of solution. Only the solution in contact with the substrates takes part in the epitaxial growth and any other volume is merely filling space. Reducing the fdead volume" of solution offers the possibility of a considerable reduction in costs. In the arrangement illustrated in FIGS. 5 to 10, the individual supports for the substrates are a close fit in a holder which in turn is a close fit in a crucible. Thus the dead volume is kept to a minimum.

The apparatus of FIGS. 5 to 10 also uses an alternative system of immersing the substrates in the solution.

Instead of the holder being inserted into the crucible containing the solution, the holder is first inserted into the crucible while the crucible is empty, the solution then being transferred to the operative chamber of the crucible. This means the holder with loaded support members can be assembled into the crucible outside the furnace, then put into the furnace. Loading, or assembly, is easier in such circumstances.

FIG. 5 illustrates a crucible 40 containing a holder 41 having a number of support members or plates 42 mounted therein. Each support member carries a substrate 43. The holder and support members are seen in more detail in FIGS. 7 to 10, and will bedescribed later.

The crucible 40 is cylindrical in shape and has a flared or tapered skirt portion 44 at its base. The interior skirt of the portion is separated from the interior of the crucible by a diaphragm 45. A solution chamber 46 is connected to the crucible by a tapered top portion 47 which is a close fit in the skirt portion 44 forming a core joint. Extending from near the bottom of the chamber 46 is a tube 48. The tube 48 passes out through the tapered skirt portion near its junction with the crucible, just below the diaphragm 45. The tube passes up the side of the crucible to a position approximately level with the top of the crucible and then back down to enter the crucible near its base.

A further tube 49 enters through the base of the chamber 46, the upper end of the tube positioned slightly below the diaphragm 45. In use a volume of solution 50 is positioned in the chamber 46. The solution 50 can be transferred to the crucible 40 by increasing the pressure in the solution chamber to force the solution through the tube 48. This can be done by applying gas pressure through tube 49. In an alternative arrangement the pressure in the crucible can be reduced, as by suction through a further tube not shown. Normally the crucible is closed by a cap-the cap conveniently carried by the cover of the furnace, or the end of the furnace tube presses down on the stack of substrates.

Considering the holder 41 and support members 42 in more detail, FIGS. 6 and 7 illustrate the holder. The holder is cylindrical and a fairly close fit in the crucible 40 with just a small clearance. The holder has a solid base 51 but the walls are divided by two slots 52 extending the full height of the holder. These slots allow solution to enter the holder as will be explained.

The support members 42 are a close fit in the holder 41 and as each support member receives a substrate slice it is stacked in the holder. The support members,

are stacked one upon the other. As will be seen in FIGS. 8 and 9, the support members are recessed at 53 on one side to receive a substrate. Two slots 54 are formed, in opposition, and the slot extend downward from the top surface of the support member to a position approximately level with the upper surface of the substrate when in position. As an indication of one size of support member which has been used, the recess was of a diameter to accept a 30m/m diameter substrate slice, the recess 1,25 mm deep, the slots 54 l.0mm deep for a slice 025mm thick. The overall thickness of a support member was 1.5 mm. These dimensions are an example only and can be varied.

The stacking of the support members 42, each with a substrate 43, in the holder 41 is illustrated in FIG. 10. The slots 54 are in alignment with the slots 52 of the holder 41. When assembled as in FIG. the holder is then slid into the crucible 40 with the base 51 resting on the diaphragm 45 as seen in FIG. 5. Preferably one of the slots 52 in the holder 41 is aligned with the end of tube 48 where it enters the crucible 40, at 55. On transfer of the solution 50 from the chamber 46, the solution flows through the tube 48 and into the crucible 40 through entrance 50. The solution flows up into the crucible to some extent by means of the small clearance between holder 41 and the crucible 40 and mainly by flowing up one of the slots 52. The solution flows into the recesses of the support members 42 via the slots 54. Solution can flow right across a support'memher and then up the second slot 52 to flow into further support member recesses. The arrangement of a small clearance between holder and crucible and the provision of slots 52 in the holder and slots 54 in the support members provides for quick and easy flow of the solution.

The assembled crucible, holder and support members, with solution in the chamber 46 is positioned in a furnace and conveniently, as previously described, the cover of the furnace carries a cap which rests on top of the crucible 40 and has a boss which presses down onto the support members to hold them in position, or the end of the furnace tube bears directly on the stack of substrates.

Although the apparatus of FIG. 5 has been described as for use with the support members and substrates in 6 with the substrates in vertical planes, it is necessary to provide some locating means to hold the substrates against the bottom surfaces of the recesses in the support members. With such an arrangement the solution chamber would still be below the crucible and the crucible and chamber would have their axes at an angle to each other.

The arrangement of FIGS. 5 to 10 provides for easy and efficient loading, reduces the dead volume of solution to a minimum and also enables the supply of solution to the substrates to be made after the apparatus and solution has been positioned in the furnace and brought to a common temperature. Also, if desired, after a predetermined time of the solution being in contact with the substrates, the solution can be caused to flow back into the chamber 46 by causing the pressure in the chamber to be lower than that in the crucible, as by applying suction to the tube 49. The solution will be kept clean and reusable.

The support members 42 can be made with recesses to accept the largest diameter of substrate to be processed, but they will accept any diameter below maximum. This enables substrate slices read to looser tolerances to be used, which reduces basic costs. The. substratesare held in the recesses by the surface tension of the solution. More than one substrate can be positioned in a recess.

It has been found that a reduction of approximately percent in the dead volumehas been obtained with the apparatus of the present invention and this can represent a 60 percent reduction in material costs for the solution. In a normal dipping process the solution is open to the furnace until the holder is inserted into the crucible and usually the solution left after the epitaxial layers have been grown is not reusable, at least until re-processed to clean it.

The material of the crucible holder, and the support members, is selected to meet the requirements of the process, that is resistance to corrosion and avoidance of any affect on the substrate or epitaxial layer. Silica is one suitable material and platinum and carbon are others.

The invention is generally applicable to any material (element or compound) that will dissolve in a suitable solvent from which it can be recrystallized onto the surface of a suitable prepared substrate crystal.

EXAMPLES:

1. III V compounds where III Ga, Al, In and V P, As, Sb

2. III V alloys e.g., Illa), IIIb), V e.g. Ga, In ,P; III Va) Vb), In ASIPPI; IIIa) IIIb), Va), Vb) Gn, In As,, P etc.

3. Elemental semiconductors, e.g., Si, Ge

4. Some II VI and IV VI compounds, e.g., CdTe,

PbTe,

5. Oxides: Ferrites, Garnets, Transition metal oxides,

Alumina, etc.

What is claimed is:

1. Apparatus for growing an epitaxial layer on one surface of each of a plurality of substrates from a saturated solution, comprising:

a crucible;

a holder of tubular cross-section, adapted to slide into the crucible and having a close fit therewith;

a plurality of support members adapted to be stacked in the holder one above the other, each support member recessed to contain a substrate;

a solution chamber connected to the crucible and positioned below the crucible when the apparatus is in use;

a tube connecting the solution chamber to the interior of the crucible;

means for causing the transfer of a saturated solution from the solution chamber to the crucible.

the holder having a base, a wall extending from the base and at least one slot extending up the wall from said base, whereby solution flowing into the crucible can flow through said slot into said holder; and

each support member having a close fit in the holder and at least one slot extending from the recess to the periphery of the support member, the support members adapted to be stacked in contact with each other and the slot in each support member arranged to align with the slot in the holder, whereby solution can flow into each recess and over the substrate therein through the slot in the holder and the slot in the support member. 2. Apparatus as claimed in claim 1, wherein the crucible and the holder are cylindrical.

3. Apparatus as claimed in claim 1, wherein the means for causing transfer of the solution comprises a tube for applying a pressure to the solution in the solu tion chamber. 

1. Apparatus for growing an epitaxial layer on one surface of each of a plurality of substrates from a saturated solution, comprising: a crucible; a holder of tubular cross-section, adapted to slide into the crucible and having a close fit therewith; a plurality of support members adapted to be stacked in the holder one above the other, each support member recessed to contain a substrate; a solution chamber connected to the crucible and positioned below the crucible when the apparatus is in use; a tube connecting the solution chamber to the interior of the crucible; means for causing the transfer of a saturated solution from the solution chamber to the crucible. the holder having a base, a wall extending from the base and at least one slot extending up the wall from said base, whereby solution flowing into the crucible can flow through said slot into said holder; and each support member having a close fit in the holder and at least one slot extending from the recess to the periphery of the support member, the support members adapted to be stacked in contact with each other and the slot in each support member arranged to align with the slot in the holder, whereby solution can flow into each recess and over the substrate therein through the slot in the holder and the slot in the support member.
 2. Apparatus as claimed in claim 1, wherein the crucible and the holder are cylindrical.
 3. Apparatus as claimed in claim 1, wherein the means for causing transfer of the solution comprises a tube for applying a pressure to the solution in the solution chamber. 